Matteo, PIerre, Yuefan and Yuhang
We discussed about the possible interaction between the SHG control loop and the Filter Cavity control loop to explain the 6.56 kHz oscillation at the error signal of the FC control loop.
If noise appears on the SHG control loop, the power level of the green beam will vary.
The gain of the FC control loop is power beam dependent and can vary.
As the FC control loop is controlled to 0 crossing of the error signal, a variation of gain has a 2nd order influence.
But if the mixer produces an offset, this influence increases with the level of this offset.
Things to do:
0) Measure the FC loop offset (at EPS1 divided by 26.5)
1) Inject a DC into PERTURB to see the influence on the oscillation
2) Measure noise in SHG without and with FC control loop
3) Match the impedance (50 Ohm) at output of the LP filter (set at the output of the FC demodulation miser)
4) Measure LO and RF levels on FC demodulation mixer
5) Remove cables between LP filter and the input of the Stanford Research SR560 amplifier (connect directly Mixer, filter and SR560 without cable)